This application requests funds for the upgrade of a pulsed electron paramagnetic resonance (EPR) spectrometer, a Bruker ESP 380E purchased in 1993, in order to provide continued support for seven research projects with NIH support involving researchers at six different institutions. We seek to update this instrument to the level of a new state-of-the-art ElexSys 580 by obtaining a modern console and a microwave bridge with ENDOR and ELDOR accessories.For each user group, the EPR measurements are either a specific aim of their NIH grants or the EPR measurements will resolve a problem on the critical path in the research. The requested update will have clear impact on all of the major users as detailed in the individual user descriptions. The benefits include: Increased sensitivity in systems where the repetition rate is limited by the current spectrometer hardware; Increased sensitivity and resolution from the elimination of coherent noise and 'Tx' noise; Elimination of current hardware limitations that produce blank 'windows' in middle of important regions of data; Increased resolution in ENDOR, TRIPLE and ENQOR spectra where resolution is currently limited by hardware limitations on pulse width; Better utilization of overnight and weekend runs resulting from computer control of many spectrometer controls that now require manual adjustment; It will make it possible to employ pulse sequences and make measurements that are currently prevented by arbitrary limitations in the hardware and software of the spectrometer.